doc: document the state of NetBSD in the Go 1.9 release notes

Updates #20836
Updates #19339
Updates #19652
Updates #20835
Updates #16511
Updates #10166
Updates #8574

Change-Id: If9a7f560489f1a8d628dafab227925bd8989326e
Reviewed-on: https://go-review.googlesource.com/47036Reviewed-by: Ian Lance Taylor <iant@golang.org>

doc/go1.9.html

@@ -108,6 +108,16 @@ type T1 = T2
   Any help in solving this FreeBSD-specific issue would be appreciated.
 </p>
 
+<p>
+  Go stopped running NetBSD builders during the Go 1.9 development
+  cycle due to persistent hangs and kernel crashes, up to and
+  including NetBSD 7.1. Any help investigating or solving the
+  <a href="https://github.com/golang/go/labels/OS-NetBSD">various
+  NetBSD bugs</a> would be appreciated. In the meantime, we can't
+  recommend running Go on NetBSD.
+</p>
+
+
 <h2 id="tools">Tools</h2>
 
 <h3 id="parallel-compile">Parallel Compilation</h3>